This invention relates generally to ultrasonic probes for use inside the body of an animal (both human and non-human) and more particularly, but not by way of limitation, to apparatus to be removably placed in an esophagus at the end of a hollow catheter for use in detecting the velocity, by means of the Doppler effect, of an acoustic reflector within the range of the apparatus.
It has been described that the use of an esophageal probe fitted with an ultrasonic Doppler transducer is an ideal way of determining the velocity of blood flow in the descending aorta. See, F. A. Duck, C. J. Hodson, and P. J. Tomlin, "An Esophageal Doppler Probe for Aortic Flow Velocity Monitoring, " Utl. in Med. and Biol., 1:233-241, Pregamon Press, Great Britain, 1974; R. E. Daigle, C. W. Miller, M. B. Histand, F. D. McLeod, and D. E. Hokanson, "Nontraumatic Aortic Blood Flow Sensing by Use of an Esophageal Probe," J. of App. Physiol., 38(6):1153-1160, June 1975; M. K. Wells, M. B. Histand, J. T. Reeves, I. E. Sodal and H. P. Adamson "Ultrasonic Transesophageal Measurement of Hemodynamic Parameters in Humans," Biomed. Sci. Instr., 14:7-12, 1978; and M. B. Histand, R. A. Corace, and M. K. Wells, "Ultrasound Doppler and Echo Combined as a Noninvasive Flowmeter," Biomed. Sci. Instr., 17:73-78, 1981. Because the descending aorta passes through the chest cavity and enters the abdominal cavity parallel to the esophagus, the proximity of the esophagus and its orientation with respect to the descending aorta are ideal for the use of Doppler ultrasound to quantify noninvasively the blood flow velocity in this area. This provides, for example, a relatively simple technique for obtaining descending aorta blood flow velocity from a medical patient during surgery, which information can be used by the anesthesiologist in controlling the anesthetic agents given to the patient, thereby providing better controlled anesthesia.
Various types of ultrasonic transducers have been incorporated into or used with a standard esophageal stethoscope for this purpose. Some have used a pulsed ultrasonic signal; however, there have been some disclosures of at least the possibility of using a continuous wave ultrasonic signal (U.S. Pat. No. 4,354,501 to Colley et al.). With a continuous wave signal, the velocity is, of course, continuously measured; additionally, use of a continuous wave signal does not require any type of time delay to separate transmitted from received signals as might be necessary in a pulsed type of probe having a single transducer for both transmitting and receiving ultrasonic signals.
Some previous types of probes have also been directional in that the transmitted ultrasonic signal was directed within a sector rather than in a complete circumferential pattern. This requires that a person try to accurately aim the signal after the probe has been placed in the patient's esophagus, thereby creating a possibly significant chance of human error that would result in inaccurate data and thus, in the example of use by an anesthesiologist, improper administration of anesthetic agents.
Some types of probes have also been disclosed as utilizing reflective surfaces in reflecting the transmitted and received ultrasonic signals at angles relative to the longitudinal axis of the probe. This is disclosed in U.S. Pat. Nos. 4,142,412 and 4,237,729, both to McLeod et al. These patents, however, teach that the transmitted and received signals must be along non-parallel paths.
Despite the existence of ultrasonic transducers used with standard esophageal stethoscopes for obtaining the same type of data which preferred embodiments of the present invention are intended to obtain, an improved omnidirectional ultrasonic probe is needed to provide accurate operation with a design which is relatively simple and economical to manufacture and use. The design should allow the steering angle of the transmitted ultrasonic signal to be easily controlled or defined. The design should utilize a continuous wave ultrasonic signal to provide for constant monitoring; but such constant monitoring should be obtained with relatively little cross-talk between the continuous transmissions and receptions. The design should include acoustic isolation and matching elements and a liquid filled chamber for enhancing the transmission and reception of the ultrasonic signals. The design should also accomodate relatively easy insertion of the apparatus within the patient. More generally, the design should be one which works and which can be readily implemented.